1. Field of the Invention
The disclosure relates generally to a system and method for offshore floating structures for use in the oilfield and related industries for exploration and extraction of minerals and resources from below surface waters. More specifically, the disclosure relates to a system and method for tension leg platforms for offshore exploration and production.
2. Description of the Related Art
With the significantly increasing demand on the oil and gas supply, offshore exploration and production from reservoirs has become vital to such supply. These reservoirs usually require large drilling rig and drilling variable payload which result in very large topsides in both size and weight. Spars and Tension Leg Platforms (“TLPs”) are the only two proven dry tree hull forms in water depths below 1524 m (5,000 ft). Each design has advantages.
A Spar platform is a type of floating oil platform typically used in very deep waters and is among the largest offshore platforms in use. A Spar platform includes a large cylinder or hull supporting a typical rig topsides. The cylinder however does not extend all the way to the seafloor, but instead is moored by a number of mooring lines. Typically, about 90% of the Spar is underwater. The large cylinder serves to stabilize the platform in the water, and allows movement to absorb the force of potential high waves, storms, or hurricanes. Low motions and a protected center well also provide an excellent configuration for deepwater operations. In addition to the hull, the Spar's three other major parts include the moorings, topsides, and risers. Spars typically rely on a traditional mooring system to maintain their position. Spars can be used in any suitable depth of water and are especially suited for deep water and ultra deep water.
A Tension-Leg Platform (“TLP”) or Extended Tension Leg Platform (ETLP) is a vertically moored floating structure normally used for the offshore production of oil or gas, and is currently suited for water depths less than 1500 meters (about 4900 ft). The hull is a buoyant structure with a topsides deck at its top that supports the exploration and/or production operations of extracting minerals, including oil and gas, and can include living quarters. Pontoons and columns provide sufficient buoyancy to maintain the deck above the waves during all predicted sea states. The platform is permanently moored by means of tendons (also known as tethers or tension legs) coupled at each of the structure's corners. These tendons are drawn tight and pull the floating TLP downward to a depth, such that the buoyancy of the TLP maintains tension on the tendons, such that virtually all vertical motion of the platform is eliminated, creating a “stiff” structure. This stiffness allows the platform to have the production wellheads on deck (connected directly to the subsea wells by rigid risers), instead of on the seafloor. The relative fixed vertical position allows a simpler well completion and gives better control over the production from the oil or gas reservoir, and easier access for downhole intervention operations.
The typical TLP couples the spacing of the columns to a supporting pontoon structure underneath the columns. The columns are coupled at the intersection of the several pontoons at the corners, forming the pontoon structure. The intersection creates a substantially rigid structural base to couple the columns. A plurality of tendons downwardly project from this intersection. To provide the needed stability to wave and wind motion, the columns are spaced apart to meet certain design criteria. However, the spacing required for the stability of the structure is often not required for support of the topsides deck on the column. Thus, the cost and extra weight of the structure can increase.
In 2005, two large hurricanes, Katrina and Rita, occurring within one month of each other in the Gulf of Mexico, United States, caused a significant loss of life, loss of platforms, and loss of production. Hurricane Katrina was the costliest hurricane, as well as one of the five deadliest, in the recorded history of the United States. Hurricane Rita hit one month later and was the fourth most intense Atlantic hurricane to hit the United States. The design criteria used at that time for offshore platforms had not predicted such stress loading on many of the existing platforms. The hurricanes destroyed over one hundred offshore platforms and damaged many others. In at least one instance of a floating TLP, the tendons ripped loose from the anchors or simply broke on one or more corners, leaving the remaining tendons taught, and causing the platform to overturn and capsize. As a result, the American Petroleum Institute and the industry in general revised its criteria for design analysis in this region for metocean criteria in the Gulf of Mexico and require additional stability and resistance to adverse movement under varying weather conditions.
U.S. Pat. No. 6,447,208 attempts to address increased stability of a TLP by adding radial wings or arms to the structure. These radial wings or arms are aligned with a vertical axis center point of the TLP and extend outwardly along a radian from each column. The patent discloses an extended-base tension leg substructure, where the substructure includes a plurality of support columns disposed about a central axis of the substructure and interconnected by at least one pontoon. Each column comprises an above water and a submerged portion. The substructure also includes a plurality of wings or arms radiating from the columns and/or the pontoons, each wing fixedly or removably securing at least one tendon extending from a wing to an anchor on the seabed. However, it is believed that the design of such radial wings form a discontinuous transition and are at a non-parallel angle with the pontoons and can create a failure mode between the radial wing and the pontoon assembly or adjacent column.
There remains a need for a different system and method for a TLP with increased stability.